Animal Diversity: Introduction to Invertebrates and Body Plans

Animal Diversity: Introduction to Invertebrates and Body Plans

§32.1: What is a “Metazoan”?

Metazoans, or animals, are a diverse group of eukaryotic, multicellular organisms that share several defining characteristics. Understanding these features is foundational for studying animal diversity.

• Eukaryotic: Cells contain a nucleus and membrane-bound organelles.

• Multicellular: Composed of multiple cells with specialized functions.

• Chemoheterotrophic: Obtain energy and carbon by ingesting other organisms.

• Life Cycle: All individuals are diploid; only gametes (sperm and eggs) are haploid.

• No Cell Walls: Instead, animals use extracellular structural proteins, especially collagen (the most abundant protein in the human body).

• Feeding: Most animals ingest food and then digest it internally.

§32.3: Basic Body Plans

Animal body plans describe the general structure and organization of an organism. These plans are shaped by evolutionary history and development.

• Symmetry: Animals may have radial symmetry (body parts arranged around a central axis), bilateral symmetry (right and left halves are mirror images), or be asymmetric.

• Body Axes: Terms such as dorsal (back), ventral (belly), anterior (head), and posterior (tail) are used to describe body orientation.

§32.1 & §32.3: Very Early Animal Development

Animal development begins with fertilization and proceeds through a series of stages that establish the body plan.

• Zygote: Fertilized egg cell.

• Cleavage: Series of rapid cell divisions without growth.

• Blastula: Hollow ball of cells.

• Gastrulation: Formation of germ layers (endoderm, ectoderm, and in some, mesoderm).

• Diploblasts: Animals with two germ layers (e.g., cnidarians).

• Triploblasts: Animals with three germ layers (e.g., most bilaterians).

§32.3: Body Cavities

Triploblastic animals may develop a body cavity, which is important for organ development and function.

• Acoelomate: No body cavity (e.g., flatworms).

• Pseudocoelomate: Body cavity not fully lined by mesoderm (e.g., nematodes).

• Coelomate: True body cavity fully lined by mesoderm (e.g., annelids, mollusks).

§32.4: Animal Phylogeny & Body Plan

Animals are classified based on evolutionary relationships and shared features. Not all animal phyla are shown in typical phylogenetic trees; only a subset is commonly depicted.

• Major Features for Categorization:

  • Presence of tissues

  • Body symmetry

  • Body segmentation

  • Molting exoskeleton

  • Embryonic tissue layers

  • Digestive openings

  • Mouth development (mouth first or second)

  • Presence of a body cavity

§32.4: Invertebrates and Vertebrates

Most animal phyla are invertebrates (animals without a backbone). The term "invertebrate" is a convenient, though not always phylogenetically precise, way to refer to these animals.

• Vertebrates: Animals with a backbone (a subgroup of chordates).

• Invertebrates: All other animals, including sponges, cnidarians, mollusks, annelids, arthropods, etc.

§33.1: Porifera (Sponges)

Sponges are among the simplest animals, lacking true tissues and organs. They are important for understanding the early evolution of multicellularity.

• Structure: No true tissues, but have specialized cells (choanocytes and amoebocytes).

• Body Plan: Asymmetric.

• Feeding: Filter feeders; water flows through pores into a central cavity.

• Reproduction: Most are hermaphroditic (often sequentially).

• Movement: Adults are sessile; larvae are motile.

• Similarity to Protists: Choanocytes resemble choanoflagellates, suggesting a close evolutionary relationship.

§33.2: Cnidarians

Cnidarians are the first eumetazoans (animals with true tissues), but have only two germ layers. They include jellyfish, corals, and sea anemones.

• Body Forms: Polyp (sessile) and medusa (motile).

• Symmetry: Radial.

• Nervous System: Simple nerve net, no brain.

• Digestive System: One opening (gastrovascular cavity).

• Specialized Cells: Cnidocytes (stinging, entangling, gripping).

• Classification: Anthozoa (corals, anemones) and Medusozoa (jellyfish, hydra).

• Corals: Secrete calcium carbonate skeletons, forming coral reefs.

§33.3: The Lophotrochozoans

Lophotrochozoans are a diverse group of bilaterian animals, including flatworms, mollusks, and annelids. They are united by molecular data and some developmental features.

Flatworms (Platyhelminthes)

• Body Plan: Bilateral symmetry, acoelomate, three germ layers.

• Digestive System: One opening (except tapeworms).

• Nervous System: Simple, with longitudinal nerve cords.

• Examples: Planaria (notable for regeneration), trematodes (flukes), tapeworms (parasitic).

Mollusks

• Body Plan: Coelomate, soft-bodied, often with a hard shell.

• Nervous System: More complex than flatworms.

• Examples: Snails, clams, squids, octopuses.

Annelids (Segmented Worms)

• Body Plan: Segmented, coelomate.

• Circulatory System: Closed, with dorsal and ventral vessels.

• Digestive System: Specialized, not segmented.

• Examples: Earthworms, leeches, polychaetes.

Table: Comparison of Major Animal Groups

Group	Symmetry	Tissues	Body Cavity	Digestive Openings	Examples
Sponges	Asymmetric	No	None	0	Bath sponge, tube sponge
Cnidarians	Radial	Yes (2 layers)	None	1	Jellyfish, coral, hydra
Flatworms	Bilateral	Yes (3 layers)	None	1	Planaria, tapeworm
Mollusks	Bilateral	Yes (3 layers)	Coelomate	2	Snail, clam, squid
Annelids	Bilateral	Yes (3 layers)	Coelomate	2	Earthworm, leech

Additional info: The notes also discuss the concept of "worms" as a general body shape, not a taxonomic group, and highlight the importance of regenerative medicine studies using animals like Hydra.